A fuel cell employing polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes formed on both sides of the polymer electrolyte membrane, i.e., an anode and a cathode. The electrode usually comprises a catalyst layer which is composed mainly of carbon particles carrying a platinum group metal catalyst and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outer surface of the catalyst layer.
Gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane disposed therebetween so as to prevent a fuel gas and an oxidant gas supplied to the electrodes from leaking out or prevent these two kinds of gases from mixing with each other. The gaskets are combined integrally with the electrodes and polymer electrolyte membrane beforehand. This is called “MEA” (electrolyte membrane-electrode assembly). Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series. At a portion thereof to come in contact with the MEA, the separator plates have gas flow channels for supplying reactant gases to the electrode surfaces and for removing a generated gas and excess gas. While the gas flow channels may be provided separately from the separator plates, grooves are usually formed on the surfaces of the separator plates to serve as the gas flow channels.
In order to supply a fuel gas and an oxidant gas to these grooves, it is necessary to use piping jigs which branch respective supply pipes for the fuel gas and oxidant gas according to the number of the separator plates to be used, and which connect the branches directly to the grooves of the separator plates. This jig is called “manifold” and the above-described type of manifold, which directly connects the fuel gas and oxidant gas supply pipes to the grooves of the separator plates, is called “external manifold”. There is also another type of manifold, called “internal manifold”, which has a more simple structure. In the internal manifold, the separator plates with the gas flow channels formed thereon are provided with through holes which are connected to the inlet and outlet of the gas flow channels such that the fuel gas and oxidant gas are supplied directly from these holes.
Since the fuel cell generates heat during operation, it needs to be cooled with a cooling water or the like to maintain the cell in good temperature conditions. Normally, a cooling section for flowing the cooling water is generally provided for every one to three cells. There are two types of configurations for the cooling section: one in which the cooling section is inserted between the separator plates; and the other in which the cooling section is formed by providing the backsides of the separator plates with cooling water flow channels, and the latter configuration is more often employed. In a general structure of a fuel cell stack, the MEAs, separator plates and cooling sections as described above are alternately stacked to form a stack of 10 to 400 cells, and the resultant cell stack is sandwiched by end plates with a current collector plate and an insulating plate interposed between the cell stack and each end plate, and is clamped with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separator plates need to have high conductivity, high gas tightness with respect to the fuel gas, and high corrosion resistance to oxidation/reduction reactions of hydrogen/oxygen, i.e., acid resistance. For such reasons, conventional separator plates have been produced, for example, by forming gas flow channels on the surface of a glassy carbon plate or a resin-impregnated graphite plate by cutting, or by putting an expanded graphite powder together with a binder in a pressing die which has grooves forming the gas flow channels formed therein, followed by press working and the subsequent heat treatment.
As described above, in the case of the method in which the separator plate is produced by cutting a glassy carbon plate, a resin-impregnated graphite plate or the like, the material cost itself for the glassy carbon plate or resin-impregnated graphite plate is high, and moreover, it is difficult to reduce the cost for cutting these materials. In the case of the method involving the press working of expanded graphite, it is difficult to improve the mechanical strength of the material, and particularly, when it is used as the power source for an electric vehicle, crack may occur due to vibration or impact during traveling. Additionally, there is the problem of difficulty in eliminating the gas permeability.
Moreover, such carbon separator plates have poor wettability with product water formed by the electrode reaction in the cell, because graphite used as the conductive agent is inherently hydrophobic. This results in a so-called flooding problem in which the gas flow channels on the surfaces of the separator plates are clogged with product water. When a large number of fuel cells are connected in series, the gas distribution among the stacked cells becomes nonuniform because the surfaces of the separator plates have poor wettability with product water, leading to another problem of variations in performance.
The polymer electrolyte fuel cell is generally used at an operating temperature of 50 to 100° C., preferably 70 to 90° C. for the purpose of reducing the specific resistance of the polymer electrolyte membrane to maintain a high power generation efficiency. The specific resistance of the polymer electrolyte membrane is reduced by humidifying the membrane to saturation so that the membrane functions as a hydrogen ion-conductive electrolyte. Accordingly, in order to maintain the power generation efficiency of the fuel cell, it is necessary to maintain the water content of the membrane in a saturated condition. For this purpose, there has been adopted a method for preventing the dehydration of the membrane, in which method water is supplied to reactant gases and the reactant gases having increased water contents are supplied to the fuel cell to suppress water evaporation from the membrane into the gases.
However, water is produced as a reaction product during the power generation by the fuel cell, and the reaction product water is discharged to the outside of the fuel cell, together with an excess reactant gas. This results in a difference between the amount of water contained in the reactant gas on the upstream side and that on the downstream side of the gas flow within the cell; accordingly, the amount of water contained on the downstream side, i.e., the outlet side, of the reactant gas is greater than that contained on the upstream side, i.e., the inlet side, by the amount equivalent to the reaction product water.
For this reason, when the reactant gases humidified to saturation are supplied to the cell in order to maintain the water content of the membrane in a saturated condition, water vapor becomes supersaturated on the outlet side and thus turns to water droplets to be mixed in the gases. Moreover, the water vapor may accumulate, as water droplets, at the inside of gas distribution grooves which serve as the reactant gases flow channels of the separator plates, and may further blockade the channels to impede the flows of the gases, resulting in supply shortage of the reactant gases, degradation of the cell performance and the like.
In addition, when the gas distribution grooves have a large width, there is the possibility that the MEA may be deformed to dangle in the gas flow channel to blockade the gas distribution grooves thereby impeding the flows of the gases, leading to supply shortage of the reactant gases, degradation of the cell performance and the like.
As the polymer electrolyte membrane, which serves the major function in the polymer electrolyte fuel cell, an ion-exchange membrane is currently employed. The ion-exchange membrane has the property of capturing any metal ions present by replacing them with protons contained in the membrane. The number of protons migrating through the membrane is decreased to increase resistance to ionic conduction when the metal ions are captured, resulting in the performance degradation of the fuel cell itself. For this reason, the fuel cell is designed to have a structure in which no metal ion reach the electrolyte membrane.
In a system employing the fuel cell, a coolant is usually carried from a fuel cell stack to an external heat exchanger via pipes, and it is then introduced to the fuel cell stack again. In general, the heat exchanger is made from a material with high thermal conduction, i.e., a metal material such as copper or aluminum. Such materials are susceptible to corrosion. Therefore, corrosion is likely to occur especially when the coolant is water, and the metal ion concentration in the cooling water is increased to such an extent that it cannot be ignored.
A separator plate comprising a mixture of resin and carbon has extremely fine gaps between carbon particles, and thus is not completely gas tight. For example, the nitrogen permeability coefficient thereof is about 1×10−16 to 2×10−15 mol/m·s·Pa.
Accordingly, when the separator plate comprising a mixture of resin and carbon is provided with a coolant flow channel, the coolant and metal ions dissolved therein permeate the wall of the separator plate little by little, although in an extremely small amount. It is particularly noted that the temperature is elevated in the fuel cell during operation. Moreover, a certain degree of pressure is applied to circulate the coolant in the narrow coolant flow channel. Furthermore, in many cases, the pressure applied to the fuel gas and oxidant gas sides is lower than that applied to the coolant, so that a force is exerted to extrude the coolant to the gas flow channel side. Consequently, the amount of the coolant permeating the separator plate is increased.
When the coolant leaches out to the gas flow channel side through the fine gaps or pores present in the molded body of carbon, excessive humidification occurs if the coolant is water, which may in some cases produce water droplets to inhibit the gases from flowing smoothly. If the coolant is oil or the like, it adheres to the electrode surfaces to produce undesirable effect to the fuel cell, such as inhibition of the functions of the electrode.
The fuel cell has a long useful life of about 5 to 10 years when used in cogeneration systems and the like. Therefore, even an extremely small amount of coolant permeation can cause impurities, such as metal ions contained in the coolant, to enter into the gas flow channels through the coolant flow channels over a long period of time and to be eventually taken into the polymer electrolyte membrane, thereby leading to degradation in performance.
Since the coolant is circulated in substantially the entire fuel cell stack, the components in contact with the coolant have a considerable degree of potential with respect to the coolant by the power generation of the fuel cell itself. While such potential varies among the sites of the fuel cell stack, if the coolant is tonically conductive, the potential is sufficient to induce corrosion due to some sort of an electrochemical reaction even in the case of a stack of several tens of cells; accordingly, the possibility is extremely high that the components will leach out and corrode in some way. Such phenomenon cannot be ignored even in the case of a separator plate comprising carbon and resin.
Thus, it is extremely important to control the ionic conductivity of the coolant, and therefore, when water is used as the coolant, an ion-exchange membrane may be installed in the circulating network of the cooling water for the purpose of suppressing an increase in the ionic conductivity, which is inevitably increased during operation of the fuel cell system. However, such method is less than perfect because an elevated temperature of the cooling water provides a stringent condition for the use of the ion-exchange membrane and thus poses the problems relating to performance, durability and the like, as well as the disadvantage such that the ion-exchange membrane has to be replaced periodically.
It is an object of the present invention to improve a conductive separator plate comprising a molded body comprising a conductive carbon at least in a portion of the gas flow channels, thereby preventing nonuniform gas distribution among the cells due to the accumulation of product water or humidifying water in the gas flow channels of the separator plates and the resulting variations in performance.
It is another object of the present invention to provide means for effectively discharge water droplets generated on the downstream side of the grooves forming the gas flow channels to the outside of a fuel cell, thereby realizing a polymer electrolyte fuel cell which permits a stable and uniform distribution of the reactant gases.
It is still another object of the present invention to prevent the coolant from permeating through the separator plate from the coolant flow channel side to leach out to the gas flow channel side, thereby inhibiting metal ions and the like contained in the coolant from reaching the electrolyte membrane to degrade the cell performance.